The present invention relates generally to processes for preparing .alpha.-cyanoacrylates. In particular, the present invention relates to a process for reacting a cyanoacetate with formaldehyde in the presence of a compound having phase transfer catalytic activity (hereinafter sometimes referred to as phase transfer catalyst) as a catalyst.
Various processes for preparing .alpha.-cyanoacrylate monomers which are widely known as base material for instantaneous adhesives have been proposed.
It is well known that processes for preparing .alpha.-cyanoacrylate monomers which have been generally practiced are based on and are improvements of a process disclosed in, for example, U.S. Pat. No. 2,467,926, which issued to A. E. Ardis. A typical one of these processes comprises reacting paraformaldehyde with a cyanoacetate in a medium of methanol in the presence of piperidine as a catalyst and thereafter removing the methanol from the reaction mixture. Then, a dehydrating solvent is added to the reaction mixture to azeotropically dehydrate the mixture, and phosphorus pentoxide is added to the resulting .alpha.-cyanoacrylate polymer (addition and condensation polymer between cyanoacetate and formaldehyde (hereinafter simply referred to as polymer) to depolymerize it, thereby producing .alpha.-cyanoacrylate monomers.
The above described process has been generally practiced, and piperidine is considered to be an effective catalyst particularly for activating formalin and a cyanoacetate.
This process includes addition and condensation polymerization, dehydration, and depolymerization steps. These steps are ordinarily carried out in one pot operation. Since the intermediate product is supplied to the subsequent step without being isolated and purified, contamination of the final product with impurities must be carefully avoided. The piperidine used in the typical reaction as described above is an essential component for the addition and condensation step, but it is an unnecessary component in the subsequent steps. When the piperidine remains in the subsequent steps, it may have an adverse effect on the dehydration and depolymerization steps. Although most of the piperidine is distilled off during the removal of methanol by distillation, the remaining piperidine is generally brought into the depolymerization step. This piperidine interferes with the depolymerization reaction and also results in a remarkable reduction in the quality and stability of the resulting crude monomer. In order to avoid these disadvantages, washing prior to the dehydration step or addition of various acids after the dehydration step has been proposed. However, these measures provided no satisfactory result.
We have previously found that when a cyanoacetate is reacted with formaldehyde in a medium of water or an organic solvent in the presence of an alkali metal hydroxide or a basic ion-exchange resin as a catalyst, good results are obtained. This process produces .alpha.-cyanoacrylate monomers of very good quality in a high yield, as compared with the conventional process using an amine or an amine-NaOH mixed catalyst in a methanol solvent. However, because in this process water is employed, the dehydration operation must be carefully conducted, which makes the operation complicated.
Hitherto, in reacting a cyanoacetate with formaldehyde, an amine such as piperidine has been generally used. Piperidine is one of the most favourable amine catalysts. However, the piperidine is not always the most effective for activating the formaldehyde or cyanoacetate from view point of a basicity. For this reason, a relatively long period of time of reaction and heating under reflux has been required, which have caused some side reactions. In addition, because in the conventional process for preparing an .alpha.-cyanoacrylate, the addition and condensation, the dehydration and the depolymerization are carried out in one pot operation and the products resulting from the respective steps are not purified, it is estimated that the catalyst or the material originating in the catalyst may cause an unfavourable behavior in the product.
In this connection, when the process we have previously discovered is carried out, the catalyst may be easily treated by washing and filtration or formation of a salt, whereby the catalyst is prevented from adversely affecting the product. In addition, because the addition and condensation can be carried out under a relatively mild condition, little side reaction occurs. However, by this process, it is difficult to conduct rapid dehydration of the polymer.
We have since conducted various experiments to overcome this difficulty and to develop improved conditions. As a result, we have found that the use of a phase transfer catalyst is very effective.